1. Field of the Invention
This invention relates to a method for removal of hydrogen sulfide from hydrogen sulfide-containing gaseous streams, such as those produced by conventional gasification processes. More particularly, this invention relates to a method for removal of hydrogen sulfide from hydrogen sulfide-containing gaseous streams in which the hydrogen sulfide is directly oxidized at low temperatures to produce elemental sulfur.
2. Description of Related Art
The oxidation of hydrogen sulfide directly to elemental sulfur has been under investigation for several years in conjunction with the conversion of toxic hydrogen sulfide to stable, non-toxic (and sometimes valuable) products, such as elemental sulfur. In recent years, some studies have been directed at the application of direct oxidation to convert the bulk of the hydrogen sulfide in synthesis gas (syngas) from gasification systems to elemental sulfur, for low-cost removal by condensation, before subsequent removal of the remaining hydrogen sulfide to very low levels by conventional or non-conventional absorption-based systems.
Prior studies of the direct oxidation of hydrogen sulfide in a synthesis gas stream having generally encountered problems with the formation of undesirable side products, such as carbonyl sulfide, COS. Furthermore, conventional wisdom has been that the temperature of the catalyst must be sufficiently high to prevent condensation of the elemental sulfur which is formed and the oxygen added must be held close to the stoichiometric levels (O2:H2S of 0.5) to prevent undesirable oxidation of valuable components of a synthesis gas, primarily CO and hydrogen.
The current state-of-the-art process for converting hydrogen sulfide to sulfur is the Claus process. In the first step of the Claus reaction, about one third of the hydrogen sulfide present in the fluid stream being processed is oxidized to sulfur dioxide, and in the second step, the remaining hydrogen sulfide and the sulfur dioxide are reacted to form sulfur. However, this reaction is limited by thermodynamic equilibrium and only a portion of the sulfur can be recovered. Sulfur recovery can be increased by using multiple stages; however, the levels of sulfur recovery efficiency required to meet current regulations are not achieved. In addition, the Claus process efficiency is affected by even small changes in the composition of the feed gas stream, temperature, or residence time. The presence of carbon dioxide in the feed can result in the formation of undesired carbonyl sulfide, which is difficult to convert to sulfur. In addition, the presence of hydrocarbons in the feed gas stream can result in catalyst deactivation and an overall reduction in process efficiency.
The selective oxidation of hydrogen sulfide to sulfur and water is an attractive method for converting the hydrogen sulfide to sulfur. The selective oxidation reaction is not limited by equilibrium and high sulfur recovery efficiencies are possible in a single stage. However, the production of sulfur dioxide as a byproduct has been a significant problem with selective hydrogen sulfide oxidation catalysts. Indeed, hydrogen sulfide can be completely oxidized to sulfur dioxide; any sulfur formed from hydrogen sulfide can be further oxidized to sulfur dioxide; and sulfur that is formed can also react with water to form hydrogen sulfide and sulfur dioxide.